1. Field of the invention
The present invention relates to antennas using a single focussing element and a plurality of feeds, and more particularly to such antennas which have a common aperture and a common boresight.
2. Discussion of Related Art
In high performance aircraft and spacecraft applications, space is usually at a premium. Yet modern systems applications frequently call for multiple, large aperture antennas with a common boresight but each having conflicting requirements (e.g. transmit/receive, widely spaced frequency bands, etc.). Consequently, there is a need to find a way to combine apertures without compromising such requirements.
An antenna boresight is defined as the beam maximum direction. For focussed antenna systems, the boresight coincides with the direction of the focal axis. Aperture is defined as the projection of the area of the focussing element on a plane perpendicular to the focal axis.
The problem of co-location and of co-boresighted apertures has been addressed in several ways in the past. Several examples of such apertures are discussed below.
Parabolic reflectors with interchangeable feeds have been suggested. This solution is similar to that of a microscope with a turret having lenses providing several discrete values of magnification. The chief disadvantage of this approach is that there are usually cables or waveguides associated with each feed which must flex or bend when a new feed is positioned to the focus. Flexing can cause phase errors or arcing problems if high power is involved. Furthermore, to minimize loss, transmitters or preamplifiers are frequently mounted on the feed which increases weight and complexity of the movable feed.
Lenses with interchangeable feeds have also been suggested. This approach is similar to the approach using parabolic reflectors with interchangeable feeds and has many of the same problems.
Frequency selective reflectors have been tried. The use of two or more apertures operating at different frequencies permits frequency selective surfaces to be used to conserve space. One example of such a reflector uses a dichroic surface subreflector positioned in front of a parabolic reflector. A first feed, near the parabolic vertex, is in the frequency band where the subreflector is reflective and so operates as a cassegrain system. A second feed is positioned at the parabolic focus and operates in the frequency band where the subreflector is "transparent." The second feed therefore operates as a point focus feed.
Another example of a frequency selective reflector system comprises a plurality of frequency selective reflectors stacked coaxially. A separate feed is directed at each of the reflectors. The first feed reflects off the first reflector, which is a bandpass surface at the frequency bands of the other feeds. Each successive feed reflects off its associated surface, which is a bandpass surface for each of the next successive feeds.
Disadvantages of the frequency selective reflector approach are that losses are associated with each frequency selective surface, particularly when the operating bands of the feeds are closely spaced in frequency. Also, losses increase and bandpass characteristics change as the angle of incidence varies. This approach trades lateral displacement of apertures for coaxial displacement and so is not very conservative of volume.
Other common boresight antennas are known. For example, U.S. Pat. No. 3,534,375 to Paine discloses a common boresight antenna for any one of several feeds. It performs this function by rotating a subreflector in a cassegrain (2 reflector) system. This system suffers from blockage of the aperture by the subreflector. This blockage is at the center of the aperture where its effect on efficiency is most severe. Also, a cassegrain antenna with a tilted subreflector and an offset feed tends to have less aperture efficiency (greater phase error) than when the feed and subreflector are coaxial and symmetrical with the main reflector, where the loss in efficiency depends on the amount of tilt and offset. Although this phase error can be compensated for to some extent in subreflector design, this correction tends to apply over a narrow frequency band.
U.S. Pat. No. 3,696,435 to Zucker discloses an antenna having a single reflector with multiple feeds; however, the feeds are not co-boresighted. Here, each feed is associated with a particular direction. Other limitations include the fact that the feeds are displaced laterally from the parabolic focal axis. Therefore, only one feed can be at the prime focus. All other feeds suffer some measure of scan loss (phase error) depending on the amount of displacement off axis. Feed locations are selected to minimize these errors, but the errors are not eliminated. Also, feed position is a function of frequency as well as lateral displacement. Thus, beam scan by rotating the reflector is not feasible.